1. Field of the Invention
The present invention relates generally to magnetic disk storage systems, and more particularly to magnetoresistive transducers for use in such systems.
2. Description of Related Art
In the magnetic recording industry, increased recording density on magnetic storage media, such as magnetic tapes, disks or the like, places an ever increasing demand on the devices employed to read the recorded information. The read transducers must have greater sensitivity and reduced vulnerability to noise and cross-talk. At the present time, the most likely candidate to meet these requirements appears to be a magnetic read sensor or head which includes a thin film magnetoresistive (MR) element.
In the past, the thin film magnetoresistive read head is formed on a rigid support substrate (which typically includes a top layer of Al.sub.2 O.sub.3 or SiO.sub.2), essentially by layering thin films of a magnetic biasing element (e.g. a magnetically "soft" alloy of NiFeCr, NiFeNb or NiFeRh), a spacer element (e.g. Al.sub.2 O.sub.3, SiO.sub.2) and a magnetoresistive element (e.g. NiFe alloy) in that order onto the substrate. The magnetoresistive element is arranged so that the magnetizing direction in the magnetoresistive element is altered upon receipt of a magnetic field signal that is written in the magnetic storage medium, and the resultant variation of an internal resistance of the magnetoresistive element, in accordance with the variation in the above magnetizing direction, is provided as an external voltage output. The soft film applies a transverse biasing magnetic field to the magnetoresistive element, such that the variation of the magnetizing direction in the magnetoresistive element is centered with reference to the direction of the biasing magnetic field and the variation of the internal resistance of the magnetoresistive element can increase or decrease with reference to the resistance at the biased point.
Proper biasing of the magnetoresistive element is critical for the linear performance of the magnetoresistive transducer. Ideally, the magnetoresistive element should be magnetically biased by the soft film such that its voltage outputs which correspond to the transitions in the magnetic field signal stored in the magnetic storage medium are symmetrical about a reference voltage. However, various factors result in asymmetry of the outputs. One of the factors is the microstructural and magnetic property variations of the soft film at a microscopic level on the wafer. Moreover, the soft film properties are not uniform across the wafer. The presence of residues and contaminants on the wafer surface before soft film deposition enhances the non-uniformities even further. All these result in non-systematic variations in the aforementioned voltage output asymmetry between manufactured magnetoresistive read heads.
The soft film microstructure and composition depends upon the surface chemistry of the underlying wafer surface on which it is deposited. For example, NiFeCr soft film reacts with the underlying Al.sub.2 O.sub.3 layer. At the contact surface, NiFeCr oxidizes in the presence of the oxide, which can take place during deposition, during subsequent processing which includes repeated annealing steps at elevated temperatures (e.g. 250 degrees C.), and during operational service lifetime due to Joule heating. Unfortunately, the surface chemistry of the wafer layer is not well defined, and is therefore uncontrollable at the atomic level.
To alleviate the above problem, sputter etch cleaning of the substrate is utilized to obtain a fresh surface before the deposition of the soft film alloy. While this approach helps to obtain a cleaner surface, it does not eliminate the problem. It is well known that the components of alloy materials do not in general have the same sputtering yield. Due to this so called "preferential sputtering" of the alloy components, a top active surface layer forms which has a stoichiometry that is different than the rest of the film and not necessarily uniform across the wafer. Longer sputter etch times to clean contaminants and to reach an equilibrium surface chemistry is not possible since this potentially induces electrical shorts through the substrate.
It has been experienced that while the soft film and magnetoresistive element thicknesses can be controlled to within 2 to 3 percent on a given wafer during deposition, variation in the voltage output asymmetry between wafers is usually 5 to 10 times higher than the wafer level thickness control. This results in a significant yield loss for the production of the magnetoresistive heads.